The shutter of a motion picture camera or projector must keep the film covered during the time when it is moving between exposures. Since it takes a given time for a continuous rotation shutter to move across a film aperture in order to cover it, and also an equal period to uncover it, these two periods must be added to the first. Only one half of the effective illumination is lost during the time of covering and uncovering, since light transmitted in the aggregate is equivalent to one period of the shutter moving across the aperture. Therefore, to have a shutter coverage during a given intermittent film transport period, for instance, 180.degree. as in a typical camera, the opaque sector or shutter blade covering angle must be 180.degree. plus the angle subtended by the aperture to result in an effective exposure of 180.degree. less the aperture covering angle.
There are a number of double shutter devices known in the art with multiple opaque sections for motion picture apparatus for the purpose of increasing light transmission. These double shutters are typically of the counter-rotating type such as shown in U.S. Pat. Nos. 2,229,237 to Brenkart and 909,627 to Roebuck. Also, there are known shutters with like rotation but with optical inversion of one to give the same effect as counter-rotation such as shown in U.S. Pat. No. 2,660,917 to Borberg.
All the foregoing counter-rotating shutters cover the light aperture more quickly with attendant gain in light transmission. However, since they cover the image edges first and the center last, then uncover the center first and the edges last, they create a very objectionable higher brightness area across the picture center.
In U.S. Pat. No. 2,260,213 to Erwin polarized shutter blades are used to obscure and uncover the aperture gradually. The object is to reduced the circular higher brightness in the aperture center, sometimes caused by illumination systems. Erwin's arrangement reduces the high brightness effect, but there is still uneven illunination distribution over the aperture as well as some brightness loss.
Finally, U.S. Pat. 2,305,201 to Sloan shows a pair of counter-rotating shutters, one with two blades of 60.degree. and one with four blades of 30.degree. rotating at half the speed for the purpose of improved light transmission. The effect is the same as that of the other discussed counter-rotating shutters: the time to cover the aperture is reduced at the expense of creating the undesirable higher brightness area across the aperture center. In addition, Sloan restricts the available aperture covering angle which requires a larger shutter diameter.